What occurs to emails in the forwarded section that experience a DMARC failure?

The Forwarded section includes emails that were sent to recipients who then automatically forwarded them to others. In such cases, SPF often fails because the forwarding server alters the sending IP, breaking SPF authentication.
To mitigate this, always set up both SPF and DKIM. DKIM stays valid even after forwarding, helping the emails pass DMARC checks.
However, even DKIM can sometimes fail when emails are auto-forwarded. This happens when the recipient's server modifies the message content or changes the email headers, breaking the DKIM signature.

If DKIM fails as well, the email’s fate depends on your DMARC policy (none, quarantine, or reject), which dictates how receiving servers should handle authentication failures. Unfortunately, there’s no way to prevent DKIM failure if the recipient’s server alters the email.